1. Field of the Invention
The present invention relates to a miniature direct current rotary electric machine.
2. Description of the Prior Art
In order to improve the efficiency of a miniature direct current rotary machine it has been proposed to use a coreless rotor therein. As such coreless rotor there have been already known and used various rotors in a form of drug-cup. Since the rotor is consituted of a coil having no iron core therein, it has many advantages that there is produced no hysteresis loss by alternate changes of magnetic flux; eddy-current loss at the side of stator is small and as a whole there is no need of worrying about iron loss or core loss.
On the other hand, however, these types of known rotary machines involve some problems in forming the coils in particular when they should be designed to satisfy specific requirements such as revolution number suitable for specific applications thereof.
For example, according to one of the known methods of winding coreless rotary coils, a plurality of curved rectangular coil elements are disposed on a rotary shaft with their centers being aligned with the center of the rotary shaft so as to form a cylindrical body. These elements assembled into a cylinder in this manner are then fixed together by a suitable bonding material such as synthetic resin applied to the circumference of the formed cylindrical body which gives a rotary coil body. This rotary coil body is relatively large in coil thickness in the direction normal to the length of the rotary shaft. Because of the large thickness, there is caused a shortage of gap magnetic flux density which produces a problem of coarse revolution.
According to another known winding method, inclined coil elements wound on a cylinder are assembled into a cylindrical body by connecting the elements at their both ends successively from field pole to field pole without any end connnection wire part between adjacent windings. The coil elements thus assembled into a cylindrical body are then fixed together by a suitable bonding material such as synthetic resin to form a rotary coil body. This coil body has a relatively small coil thickness in the direction normal to the length of the rotary shaft. However, when the coil body is desired to have a short axial length to give a flattened shape of coil, there arises a problem. The problem is that the breadth of windings, that is, the number of active conductors, is severely limited with the decrease of the angle of inclination of the active coil interlinking with magnetic flux.
To solve the above problems involved in the first and second winding systems according to the prior art, we have already proposed an improved type of coreless rotary coil body in a form of cup. The coil body comprises a coil part inclined with a predetermined angle of inclination on the circumference of the coil body and an end connection part disposed to make a connection between adjacent windings of the coil part only at one side end of the inclined coil part. This system of winding enables one to improve the rate of winding and increase the number of active conductors while reducing the resistance of winding (armature resistance). This improvement is the subject of a prior application filed by the assignor of the present invention and published as Japanese Patent Application Publication No. 22361/1974.
In designing a coreless type of direct current motor with a cup-shaped rotary armature it is essential to suitably select the direct current resistance R, the number of active conductors Z and the active magnetic flux .PHI. for the armature whose field system is a permanent magnet. As well known in the art, iron loss (core loss), copper loss (ohmic loss) and mechanical loss constitute three important losses in direct current motors. If the rotor is formed as a coreless one, then the hysteresis loss caused by the alternate change of magnetic flux is eliminated and also the eddy-current loss occurring at the side of the stator becomes negligibly small. It is no longer necessary, as a whole, to take the iron loss into account. Furthermore, by using a coreless structure, the reactance voltage usually generated in the coil at the time of commutation can be reduced to the lowest level and therefore nearly ideal commutation is attainable which gives the commutating mechanism an improved stability and an extended useful life.
For the above mentioned type of motors, the following pure equation of motor circuit in which no iron loss is taken into consideration holds well: EQU IaV-Ia.sup.2 R=IaEc . . . (1)
wherein,
V=terminal input voltage, PA1 Ia=armature current, PA1 Ec=back electromotive voltage and PA1 R=Ra+Rb in which
Ra=armature resistance and PA2 Rb=brush contact resistance.
Therefore, a larger output IaEc can be obtained by reducing the ohmic loss Ia.sup.2 R to a smaller value relative to the input IaV. This means that a motor of very high efficiency can be made by a proper control of the mechanical loss contained in IaEc.
However, there are some applications of motor for which the motor has to be designed to satisfy particular requirements regarding revolution number and other properties. In such case, a particular technique is required by which the actual values of R, Z and .PHI. can be determined most suitably for the aimed purpose.
Techniques for forming a cylindrical, cup-shaped coil without any end connection at its both ends are disclosed, for example, in Japanese Patent Application Publication No. 2151/1963, U.S. Pat. No. 3,360,668 and DAS No. 1,188,709. One example of such cup-shaped coil is shown in FIG. 2. There may be the case wherein a coil in a form of flattened cup as shown in FIG. 1 should be designed employing the technique used for the coil of FIG. 2 which has a larger axial length than that in FIG. 1. However, the use of the known technique as mentioned above for making such a flat cup-shaped coil as shown in FIG. 1 has some difficulty. The inclination .theta. of the active coil interlinking with the magnetic flux in FIG. 1 is smaller than that in FIG. 2. As shown in FIG. 3, when the inclination .theta. is small, a limitation is put on the width of coil segment So' by which the number of active conductors is determined. The limitation is sharply enhanced with the reduction of the inclination .theta..
In FIG. 3, the symbol So is a quotient given by dividing the length of circumference of the rotor by the number of commutator segments. Therefore, So means coil width per segment and the number of windings which can be wound within the width of So corresponds to the number of coils which can be wound in one slot on an iron core. Even when coils have the same width of So, the width So' within which the coil can be really wound may be different from each other. Since SO'=So.multidot.sin .theta., the width So' varies depending upon the inclination of coil .theta.. Of course, .theta. must be constant for one coil. If it varies from one place to another in one and same coil, So' will be limited by the smallest inclination .theta. in the coil. In this sense, the locus of coil turn must describe a spiral with a constant inclination on the cylindrical surface of the armature. To receive the effective number of conductors in the width So', the diameter of wire to be wound is decreased with the decrease of the axial length (cup depth) for the same diameter dm of armature. Thereby, the armature resistance Ra in the above equation (1) is increased and therefore the ohmic loss is increased which reduces the efficiency of the motor.
FIG.4 shows one example of a coil disclosed in the above mentioned our prior application Japanese Patent Application Publication No. 22361/1974. As the coil body has an end connection part provided only at its one side, the inclination of coil winding wound on a cylindrical surface, that is, the angle .theta. can be selected at will. When the axial length of the cup-shaped coil is reduced, the inclination .theta. of the active coil winding part is not reduced in proportion to the reduction of axial length but is set to an optimum value obtained by a calculation of the three important factors, armature resistance Ra, number of active conductors Z and total active magnetic flux .PHI.. Therefore, the severe limitation concerning the width So' described above can be moderated to prevent the increase of armature resistance Ra when a flat cup-shaped coil is used.
When the flat cup-shaped coil shown in FIG. 1 is compared with that shown in FIG. 4 in respect to the resistance Ra, it is found that Ra for the former is 2.23 .OMEGA. and that for the latter is 0.66.OMEGA. provided that for both the coils, dm (average diameter)=29.4 mm, lc(coil height)=18 mm, tthe number of commutator segments=5 and the number of active conductors Z=240 lines.
In case of the cup-shaped coil body shown in FIG. 4 for which the above calculation was made, the turn-back points A and C are set at the positions of .pi.dm/2, that is, the positions opposed to each other at 180.degree. and the segment AC of winding extends along the upper edge of the cup. But, this can be modified as shown in FIG. 5. In the modification shown in FIG. 5, the segment AC of the winding extends straightly or almost straightly to form a chord of the circular upper edge of the cup serving as an end connection part. Employing the modification of FIG. 5, a further reduction of the resistance Ra can be attained without any reduction of the effective values of Z and .PHI.. To demonstrate this, ABCA=10.4 cm and Ra=0.66.OMEGA. for the FIG. 4 example are compared with the data ABCA=9.2 cm and Ra=0.59.OMEGA. for the FIG. 5 example. Compared with the conventional cup-shaped coil body shown in FIG. 1, the armature resistance Ra is reduced to 1/4.
As shown in the above, the invention of Japanese Patent Application Publication No. 22361/1974 was directed primarily to analyze R and Z of the three important factors. A further development of the invention has led us to the finding that the area of the coil intersecting the magnetic flux can be increased or decreased as desired by suitably selecting the positions of end connection. This finding has been disclosed in detail in German laying-upon print DT-OS No. 2,126,199. Namely, it has been found that the turn-back points A and C mentioned above are not always necessay to lie on the diameter of the cup, that is, at such positions corresponding to .pi.dm/2, but the inclination .theta. can be decreased or increased according to the extent to which the cup should be flattened.
In the above calculation of armature resistance Ra, such case was shown in which the end connection positions lie on the dimeter of the cup (FIG. 5). The end connection positions may be shifted as seen in FIG. 8. In FIG. 8, if the position of x is displaced in the negative direction relative to X--X' axis, one can find out such position in which the area of coil becomes maximum as later shown by a numerical calculation. On the contrary, if the position x is displaced in the direction of the positive side of the X--X' axis, then the winding rate will be further improved although the area of coil will be decreased.
The following description explains the manner of how to find out the position in which the area of coil is maximum:
It is known that the quantity of any one axis component of moment generated in a closed circuit, when the closed circuit formed by any closed curve is placed in a parallel magnetic field, is in proportion to the area of orthogonal projection of the closed circuit on a plane extending parallel with the axis and the direction of the magnetic field. FIG. 6 shows the relation between the orthogonal projection coil area and the position of the end connection for a coil as shown in FIGS. 5 and 8. The central angle .alpha. to the end connection length AC is referred to as end connection angle which may be either .alpha. (narrow angle) or 2.pi.-.alpha. (wide angle). In this case, the area of coil given as follows:
As curve L is a spiral line, let k denote the tangent to the inclination of the spiral L. .gamma. is the angle shown in FIG. 6 which indicates the position of winding by an angular coordinate of cylindrical coordinate system. Then, EQU z=r.gamma.k . . . (1)
and EQU y=r sin .gamma.=r sin (z/rk) . . . (2).
Formula (2) indicates that the orthogonal projection of spiral (L) is a sine curve. Torque T(t) generated in one coil winding is given as follows by a numerical calculation through many transformations of the formula not shown: EQU T(t) max=1.45 rlBI . . . (3)
wherein, B is gap magnetic flux density in gauss and I is electric current in ampere. In the shown case, .alpha.=92.92.degree..about.92.94.degree..apprxeq.93.degree..
Since the case shown in FIG. 7 corresponds to such case in which t=1 in FIG. 6, there is given: EQU T(1)=1.27 rlBI . . . (4)
Let t=1/2 (end connection lies on the diameter of the cup), then, EQU I(1/2)=1.27 rlBI . . . (5) .
Thus, the area of coil obtained is the same for the two cases. But, since .theta. is larger in case (5) than in case (4), it is seen that the case (5) is more advantageous than the case (4) with respect to winding rate.
In summary, it may be said that for a cup-shaped armature coil there is obtained a freedom in selection of the values of R, Z and .PHI. by providing an end connection at one side end, without any loss of structural functions as a cup-shaped rotor. However, practice, this can be realized only when there is established a useful method by which a conductor wire can be wound in many turns orderly and prefectly in accordance with the above principle. Otherwise, it is impossible to wind a given number of wires having a given wire diameter into a coil of predetermined thickness. Especially, there may often occur such trouble that the crossed parts of wires are crushed by the pressing pressure applied to the parts at the coil thickness shaping step after winding and thereby a rare short is caused.
FIG. 9 is a typical characteristic curve of a direct current motor having a field system of magnet. In FIG. 9, the abscissa is torque T and the ordinate is current I for curve 1 and revolution number N for curve 2. Let K denote the torque constant and m the revolution number separation characteristic constant which is a reciprocal of ratio of the change .DELTA.n of revolution number N to a torque change .DELTA.T, then, EQU K=.DELTA.T/.DELTA.I and m=.DELTA.T/.DELTA.n
wherein, .DELTA.I is change of current I for a certain change of torque .DELTA.T.
As to the constant m, it is also known that it is in proportion to the square of K and is in reciprocal proportion to the resistance R between motor terminals (m.about.K.sup.2 /R). Therefore, if applied voltage V becomes known, all the factors such as torque required for the motor, its revolution number N, driving current I, suspension constant m and starting torque can be determined by the values of torque constant K and resistant R.
Regarding the torque constant K, it is also known that it is in proportion to the product of number of active conductors Z multiplied by total active magnetic flux .PHI. (K.about.Z.PHI.). This means that it is essential to properly select the values of Z and .PHI. in designing a motor for obtaining the desired output of revolution torque. On the other hand, in order to minimize the production cost of the motor, it is required to use an inexpensive magnet such as that of barium-ferrite system instead of an expensive alnico magnet as the field magnet in the motor. To satisfy the requirement concerning the production cost, it is inevitable that .PHI. becomes small. This reduction in .PHI. must be compensated by increasing Z, the active conductor lines per segment. In this case, to maintain a certain necessary value of m it is required to satisfy the condition that the resistance R should not be increased with the increase of Z. This condition necessitates the use of such conductor wire which is low in resistance per unit length (wire of larger diameter).
For a cup-shaped coreless revolving coil, a certain number of conductors having a certain wire diameter as given by the above calculation are wound up on a cylindrical coil body in two or more layers to form a cup coil having a desired coil thickness t. However, note should be taken or the fact that a regular relation as shown in FIG. 10 is not always established between the wire diameter d used and the coil thickness t and, instead, it is often required to wind up conductor wire in an indefinite number of layers as shown in FIG. 11. In other words, according to the prior art, the coil thickness t is limited only to an integral multiple of the diameter d of wire used at that time, which is usually twice. There has not yet been known any method which enables to form a coil with any desired coil thickness independently of the wire diameter.
Among many coil winding methods hitherto known for the above mentioned type of coreless resolving coil, the method disclosed in the Japanese Patent Application Publication No. 22361/1974 is featured in that adjacent windings are conneced with each other only at the one side end of the inclined coil part wound on the coil body as shown in FIG. 5. As clearly shown in FIG. 5, every element coil is wound in such manner that the elements form the opening of a cup of their lower ends and they are turned back at the opening toward the upper edge of the cup along the cylindrical surface of the cup. At the upper edge, adjacent windings are connected with each other successively at the end connection part.